This application relates to orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) communication systems, and more particularly to generation and transmission of preamble signals for fast cell searching, time-synchronization, and correcting initial frequency offset in an OFDM or OFDMA communication system.
OFDM and OFDMA systems may be used in various telecommunication systems, including wired and wireless communication systems, to provide various types of communication services, such as voice and data. A wireless communication system covers a certain geographic area by dividing the area into a plurality of cells, which can be further divided into two or more sectors. The base stations, which conceptually locate at the center of respective cells of their coverage, transmit information to the mobile subscriber stations (MSS) via downlink (DL) radio signals. A mobile station is also known as the mobile station (MS), the subscriber station (SS), or the wireless station. The mobile stations transmit information to their serving base stations via uplink (UL) radio signals.
The downlink radio signals from the base stations to mobile stations may include voice or data traffic signals or both. In addition, the base stations generally need to transmit preamble signals in their downlink radio signals to identify to the mobile stations the corresponding cells and corresponding segments in the cells to which the downlink radio signals are directed. Such a preamble signal from a base station allows a mobile station to synchronize its receiver in both time and frequency with the observed downlink signal and to acquire the identity, such as IDcell and Segment, of the base station that transmits the downlink signal.
IEEE 802.16 OFDMA has been developed to provide wireless communications based on an orthogonal frequency division multiple access (OFDMA) modulation technique. In the DL preambles currently defined in IEEE 802.16 OFDMA, the MSSs store predefined and handcrafted pseudo-noise (PN) like sequences for identifying IDcell numbers and segment numbers of the adjacent cells. In operation, a MSS captures the preamble symbols in received downlink signals and correlate the preamble in each received downlink signal with the stored pseudo-noise (PN) like sequences to determine IDcell and Segment of a specific sector for that received downlink signal. These preamble sequences are handcrafted in advance and are processed by the MSS one at a time. There are more than 100 such sequences in some implementations of the current IEEE 802.16 OFDMA. Performing the cross-correlation with such a large number of preamble sequences can be time consuming and increase the hardware costs. In addition, MSSs store the entire set of preamble sequences and this storage further increases the hardware cost.
One important performance parameter of the preambles is the peak-to-average-power-ratio (PAPR). To reduce the system cost, the PAPR for the preamble should be as small as possible. It is well known that OFDM usually has a relative higher PAPR ratio than other modulations. This is especially important for a preamble because the preamble is transmitted in every frame.